For over two decades, the electron microscope has been a keystone in the laboratory and epidemiologic investigations of the Laboratory of Infectious Diseases. Its use and application to uncharted areas of research led to the discovery of the Norwalk virus (the first major viral etiologic agent of diarrhea), the Hawaii virus, the Montgomery County virus and the hepatitis A virus, as well as the first detection in the United States of human rotavirus (which was discovered in Australia). Because of our inability to cultivate the Norwalk group of caliciviruses in any cell culture system and the recent expression of 27nm virus-like particles belonging to this group, the electron microscope continues to serve a seminal role in the research program of the Epidemiology Section. It is the only method for (i) the detection of the 27nm recombinant, virus-like particles, as well as the native 27nm virus particles and, (ii) the elucidation of antigenic relationships among these recombinant virus-like particles with the use of immune electron microscopy. In addition, the electron microscope provides an important adjunct to many of the studies of the section, providing information for laboratory, epidemiologic and clinical studies. The scope of the demand for electron microscopic support is evidenced by the fact that some 69 individual experiments were inducted by electron microscopy since the previous annual report.